There are six types of leptons, known as flavours, forming three generations.[2] The first generation is the electronic leptons, comprising the electron (e−) and electron neutrino (ν
e); the second is the muonic leptons, comprising the muon (μ−) and muon neutrino (ν
μ); and the third is the tauonic leptons, comprising the tau (τ−) and the tau neutrino (ν
τ). Electrons have the least mass of all the charged leptons. The heavier muons and taus will rapidly change into electrons and neutrinos through a process of particle decay: the transformation from a higher mass state to a lower mass state. Thus electrons are stable and the most common charged lepton in the universe, whereas muons and taus can only be produced in high energy collisions (such as those involving cosmic rays and those carried out in particle accelerators).

Following a suggestion of Prof. C. Møller, I adopt — as a pendant to "nucleon" — the denomination "lepton" (from λεπτός, small, thin, delicate) to denote a particle of small mass.

The etymology incorrectly implies that all the leptons are of small mass. When Rosenfeld named them, the only known leptons were electrons and muons, which are in fact of small mass — the mass of an electron (6999511000000000000♠0.511 MeV/c2)[18] and the mass of a muon (with a value of 7002105700000000000♠105.7 MeV/c2)[19] are fractions of the mass of the "heavy" proton (7002938300000000000♠938.3 MeV/c2).[20] However, the mass of the tau (discovered in the mid 1970s) (7003177700000000000♠1777 MeV/c2)[21] is nearly twice that of the proton, and about 3,500 times that of the electron.

Nearly 40 years after the discovery of the electron, the muon was discovered by Carl D. Anderson in 1936. Due to its mass, it was initially categorized as a meson rather than a lepton.[25] It later became clear that the muon was much more similar to the electron than to mesons, as muons do not undergo the strong interaction, and thus the muon was reclassified: electrons, muons, and the (electron) neutrino were grouped into a new group of particles – the leptons. In 1962, Leon M. Lederman, Melvin Schwartz, and Jack Steinberger showed that more than one type of neutrino exists by first detecting interactions of the muon neutrino, which earned them the 1988 Nobel Prize, although by then the different flavours of neutrino had already been theorized.[26]

The tau was first detected in a series of experiments between 1974 and 1977 by Martin Lewis Perl with his colleagues at the SLACLBL group.[27] Like the electron and the muon, it too was expected to have an associated neutrino. The first evidence for tau neutrinos came from the observation of "missing" energy and momentum in tau decay, analogous to the "missing" energy and momentum in beta decay leading to the discovery of the electron neutrino. The first detection of tau neutrino interactions was announced in 2000 by the DONUT collaboration at Fermilab, making it the latest particle of the Standard Model to have been directly observed,[28] apart from the Higgs boson, which probably has been discovered in 2012.

Although all present data is consistent with three generations of leptons, some particle physicists are searching for a fourth generation. The current lower limit on the mass of such a fourth charged lepton is 7002100800000000000♠100.8 GeV/c2,[29] while its associated neutrino would have a mass of at least 7001450000000000000♠45.0 GeV/c2.[30]

Leptons are spin-1⁄2 particles. The spin-statistics theorem thus implies that they are fermions and thus that they are subject to the Pauli exclusion principle; no two leptons of the same species can be in exactly the same state at the same time. Furthermore, it means that a lepton can have only two possible spin states, namely up or down.

A closely related property is chirality, which in turn is closely related to a more easily visualized property called helicity. The helicity of a particle is the direction of its spin relative to its momentum; particles with spin in the same direction as their momentum are called right-handed and otherwise they are called left-handed. When a particle is massless, the direction of its momentum relative to its spin is frame independent, while for massive particles it is possible to 'overtake' the particle by a Lorentz transformation flipping the helicity. Chirality is a technical property (defined through the transformation behaviour under the Poincaré group) that agrees with helicity for (approximately) massless particles and is still well defined for massive particles.

In many quantum field theories, such as quantum electrodynamics and quantum chromodynamics, left- and right-handed fermions are identical. However, in the Standard Model, left-handed and right-handed fermions are treated asymmetrically. Only left-handed fermions participate in the weak interaction, while there are no right-handed neutrinos. This is an example of parity violation. In the literature, left-handed fields are often denoted by a capital L subscript (e.g. e−L) and right-handed fields are denoted by a capital R subscript.

One of the most prominent properties of leptons is their electric charge, Q. The electric charge determines the strength of their electromagnetic interactions. It determines the strength of the electric field generated by the particle (see Coulomb's law) and how strongly the particle reacts to an external electric or magnetic field (see Lorentz force). Each generation contains one lepton with Q = −e (conventionally the charge of a particle is expressed in units of the elementary charge) and one lepton with zero electric charge. The lepton with electric charge is commonly simply referred to as a 'charged lepton' while the neutral lepton is called a neutrino. For example, the first generation consists of the electron e− with a negative electric charge and the electrically neutral electron neutrino ν
e.

In the language of quantum field theory, the electromagnetic interaction of the charged leptons is expressed by the fact that the particles interact with the quantum of the electromagnetic field, the photon. The Feynman diagram of the electron-photon interaction is shown on the right.

Because leptons possess an intrinsic rotation in the form of their spin, charged leptons generate a magnetic field. The size of their magnetic dipole momentμ is given by

μ=gQℏ4m,{\displaystyle \mu =g{\frac {Q\hbar }{4m}},}

where m is the mass of the lepton and g is the so-called g-factor for the lepton. First order approximation quantum mechanics predicts that the g-factor is 2 for all leptons. However, higher order quantum effects caused by loops in Feynman diagrams introduce corrections to this value. These corrections, referred to as the anomalous magnetic dipole moment, are very sensitive to the details of a quantum field theory model and thus provide the opportunity for precision tests of the standard model. The theoretical and measured values for the electron anomalous magnetic dipole moment are within agreement within eight significant figures.[31]

In the Standard Model, the left-handed charged lepton and the left-handed neutrino are arranged in doublet(ν
eL, e−L) that transforms in the spinor representation (T = 1⁄2) of the weak isospinSU(2) gauge symmetry. This means that these particles are eigenstates of the isospin projection T3 with eigenvalues 1⁄2 and −1⁄2 respectively. In the meantime, the right-handed charged lepton transforms as a weak isospin scalar (T = 0) and thus does not participate in the weak interaction, while there is no right-handed neutrino at all.

The Higgs mechanism recombines the gauge fields of the weak isospin SU(2) and the weak hypercharge U(1) symmetries to three massive vector bosons (W+, W−, Z0) mediating the weak interaction, and one massless vector boson, the photon, responsible for the electromagnetic interaction. The electric charge Q can be calculated from the isospin projection T3 and weak hypercharge YW through the Gell-Mann–Nishijima formula,

Q = T3 + YW/2

To recover the observed electric charges for all particles, the left-handed weak isospin doublet (ν
eL, e−L) must thus have YW = −1, while the right-handed isospin scalar e−
R must have YW = −2. The interaction of the leptons with the massive weak interaction vector bosons is shown in the figure on the left.

In the Standard Model, each lepton starts out with no intrinsic mass. The charged leptons (i.e. the electron, muon, and tau) obtain an effective mass through interaction with the Higgs field, but the neutrinos remain massless. For technical reasons, the masslessness of the neutrinos implies that there is no mixing of the different generations of charged leptons as there is for quarks. This is in close agreement with current experimental observations.[32]

However, it is known from experiments – most prominently from observed neutrino oscillations[33] – that neutrinos do in fact have some very small mass, probably less than 7000200000000000000♠2 eV/c2.[34] This implies the existence of physics beyond the Standard Model. The currently most favoured extension is the so-called seesaw mechanism, which would explain both why the left-handed neutrinos are so light compared to the corresponding charged leptons, and why we have not yet seen any right-handed neutrinos.

The members of each generation's weak isospindoublet are assigned leptonic numbers that are conserved under the Standard Model.[35] Electrons and electron neutrinos have an electronic number of Le = 1, while muons and muon neutrinos have a muonic number of Lμ = 1, while tau particles and tau neutrinos have a tauonic number of Lτ = 1. The antileptons have their respective generation's leptonic numbers of −1.

Conservation of the leptonic numbers means that the number of leptons of the same type remains the same, when particles interact. This implies that leptons and antileptons must be created in pairs of a single generation. For example, the following processes are allowed under conservation of leptonic numbers:

However, neutrino oscillations are known to violate the conservation of the individual leptonic numbers. Such a violation is considered to be smoking gun evidence for physics beyond the Standard Model. A much stronger conservation law is the conservation of the total number of leptons (L), conserved even in the case of neutrino oscillations, but even it is still violated by a tiny amount by the chiral anomaly.

Using the values of the 2008 Review of Particle Physics for the branching ratios of muons[19] and tau[21] yields a lifetime ratio of ~6993129000000000000♠1.29×10−7, comparable to the measured lifetime ratio of ~6993131999999999999♠1.32×10−7. The difference is due to K1 and K2 not actually being constants; they depend on the mass of leptons.